Taking a walk with the free-runners of Paris is not a relaxing way to spend the morning. While most of us stride calmly along the pavement, the free-runners leap from bollard to bollard, vaulting over benches, scaling up tree trunks. Not content with merely turning corners, they swing around lampposts. Where most pedestrians will avoid a road barrier, they jump on it, do a headstand and dangle themselves from it upside-down. Walking, in its traditional form, is dismissed as a bourgeois convention.

A new urban sport which emerged from the southern suburbs of Paris, free-running uses gymnastic skills to find alarming new ways of navigating the urban landscape. It is the free-runners' fondness for catapulting themselves at dangerous heights over anxiety-inducing distances that has brought them notoriety - initially within the confines of their mayor's office, but more recently on an international level. "I see it as a way of freeing onself from the constraints of conformist behaviour," says Sebastien Foucan, one of the sport's founders.

Next month, Channel 4 will broadcast a documentary showing Foucan and two of his proteges on a daytrip to London, hurling themselves from some of its most famous landmarks. Jump London records them dancing around the perimeters of the Albert Hall's dome, suspending themselves, head-first, from the concrete balconies of the National Theatre, somersaulting along the roofs of Somerset House and Shakespeare's Globe - without harnesses or safety nets.

Since it first emerged in the mid-90s, the art of le parkour (a corruption of the French for obstacle-course racing) has attracted a huge underground following in France, and gradually across the world. Websites in Japanese and Russian pay homage to the sport's creators and its birthplace in this otherwise wholly unremarkable suburb of Lisses.

Inevitably it has its detractors. "It's all very well getting young people off the streets," one commentator remarked. "But it's no good if they end up hurling themselves from the rooftops." The mayor of Lisses has given up trying to stop them "running up and down the walls like cats" but has issued a plea that they shouldn't perform dramatic acrobatics near the windows of old people's homes because it scares them.

To explain a few of the principles of the sport, Foucan, 29, and another of the stars of the Channel 4 film, Johann Vigroux, 20, return to the park where it was born in the early 1990s. Lisses is a quiet town 50 minutes south of Paris, situated beyond the warehouses, car-crushing plants, abandoned building sites and rubbish dumps of the more deprived suburbs. The roads are well swept and the gardens are tidy. It's a safe place to live, and the local school-children find it very dull. "There wasn't much for us to do," Foucan says. "We'd meet here after school and dare each other to do stunts in the playground. We preferred messing around to conventional games like football."

Unlike his contemporaries, Foucan and a group of around 10 schoolfriends never abandoned these games. "When most people get older they stop worrying about whether they're walking on the cracks or the squares of the pavements. We focused on it more and more." Their dedication has proved lucrative; Nike and Toyota have both hired Foucan to translate the leaps he learned on the roof of the local swimming pool into high-speed advertising.

Vigroux and Foucan refuse - on the grounds of basic safety - to teach the principles of roof-leaping straight off, preferring instead to try to explain the mentality of a free-runner. This involves following them around the deserted suburban streets, isolating potential hurdles.

"Simply walking is a wasted opportunity. What you need is a more imaginative approach to propelling yourself along the street," Foucan explains, balancing on a lump of concrete on the pavement. "You have to understand the philosophy. This is not a rock, that is not a tree, nor is that a lamppost," he says pointing first at a rock, then at a tree then at a lamppost. "I see them as launchpad, a swing and a ladder. Ideally, the street would be covered with obstacles like this." He launches himself at a wall, effortlessly walking up it, as if the centre of the earth's of gravity has shifted.

"It's a question of being creative in the way that you move," he adds, disappearing up a fire escape, springing on to the banisters and hopping to a nearby wall - apparently oblivious both to the narrowness of the wall and the drop to the asphalt beneath him. The challenge is to make potentially suicidal leaps look as easy as stepping off the curb. "You need to do thousands of small jumps before you try anything ambitious."

The local administration has erected fences on the roofs of some buildings to try to prevent them from jumping. "If they knew anything about free-running they wouldn't have bothered," Vigroux says. "It increases the challenge. Every time we see them, we say thank you to the mayor."

There are some hurdles that free-runners should avoid on principle. "We don't jump over cars anymore, unless they're our own. We prefer to not to attract the attention of the police," Vigroux says. The group is diligently law-abiding, and even their website (www.parkour.com) has a strict section on discipline. "People don't appreciate seeing youths jumping on their walls and garages," it warns. "Don't break people's windows. Don't jump on their flowerbeds and don't insult people who try to stop you."

Safety is an obsession, not least because the group knows of two people in France who died while trying to copy them shortly after the release in 2001 of a film about the sport, directed by Luc Besson. "You must start with the basic moves, work in groups and don't do anything ambitious until you have the experience behind you. A single false jump can be fatal," Foucan says. Even his team members who claim to train for several hours every day have ripped their ligaments and broken their bones. "Besides, you don't need to take risks to be a free-runner. It's much more about a way of moving." He is not embarrassed to admit that he still trains in the playground at the nearby park, ignoring the fact that it is restricted to children under 12.

Some of the childhood friendships forged here have soured as their creation gained recognition. The making of Besson's film spawned a subgroup, who call themselves Yamakasi; the free-runners will have nothing to do with them. David Belle, another founding member and the hero of a BBC advertising campaign last year, defected after an apparent clash of egos and ambitions and now has his own movement, Les Traceurs. A fierce sense of rivalry has grown between the factions, all of whom continue to use the architecture of Lisses as an outdoor gym, but despite the divisions, the popularity of their sport appears to be growing.

Most aficionados regard themselves as professional sportsmen, but it isn't, according to Foucan, a movement that is likely to receive official recognition in France. "This is a conservative country and the authorities remain very suspicious of us. They see us as people from the suburbs who've decided to swap hell-raising and violence for a sport that involves leaping all over the buildings," Foucan says. "They don't see that there's any improvement and they get scared thinking about what would happen if everyone started jumping from the balconies. We've been given a much better welcome abroad."

Don't try this at home: the basics of free-running

Blind jump The term applied to a precision jump which is undertaken in a situation which makes it impossible to see the landing spot.

Tic-tac Run up, use the foot to grip onto a small hurdle and propel yourself from it to leap with greater ease over the next obstacle.

Basic jump Launch yourself forward, landing on both feet, bending the knees to absorb the shock. The manoeuvre is completed with a somersault.

Cat jump Run towards the target, place both hands on the wall, leap through them - with the legs through the middle of the arms. Land on both feet.

Precision jump This move involves identifying a small or very precise landing target, and being able to judge the exact leap necessary.